While recovering petroleum offshore it has turned out that there is a need to exceed the technical and economic lifetime originally intended for well equipment. This holds true also of safety-critical components such as a Christmas tree with associated valves and control apparatuses.
A Christmas tree located on the seabed has often been guided onto a production tubing head via guide posts. The Christmas tree communicates with equipment on the surface via a so-called umbilical which may typically comprise cables for electrical power and signals, optical fibres for signal transmission, tubes for hydraulic fluid under high pressure and low pressure and also tubes for the supply of chemicals. It is common for the. Christmas tree to be provided with a valve jacket which constitutes a barrier between the annulus, the production passage and the surroundings. The valve jacket must be removed when a suitable tool provided with connections for fluids that are used for removing deposits and also for well-killing is to be fitted.
The terms high pressure and low pressure are not exact, as they vary between the different suppliers. Roughly, a pressure between 100 and 300 bars is termed low pressure whereas a pressure above 300 bars is termed high pressure. By pressures below 100 bars, actuators and valves may take a so-called “fail safe” position which will often shut down the petroleum production.
The hydraulic fluids are typically carried to, respectively, a high-pressure and a low-pressure valve pack on the Christmas tree, the high-pressure valve pack communicating with a downhole blowout preventer, whereas the low-pressure valve pack communicates with, among other things, a number of actuators mainly for valve control in the Christmas tree.
Electrical power and control signals are conveyed to a submerged control module on or by the Christmas tree. The control module which is controlled from the surface is connected to the different hydraulic valves of the valve packs and thereby controls the different valve functions in the Christmas tree.
Known Christmas-tree installations exhibit several weaknesses that emerge after a long operating time. The control module is prone to functional faults while, at the same time, the availability of new control modules of the kind in question and also spare parts therefor is limited. Valve leakages do occur in the so-called production-swab valve (PSV) of the Christmas-tree jacket. The monitoring here is often insufficient. Leaks into the annulus between the well and the Christmas tree do occur as well. Further, it has turned out that pressure sensors are prone to faulty functioning.
The umbilical with associated components is prone to leakages.
Considerable delays in deliveries of umbilicals and associated operations do occur as well, leading to delayed production start for the relevant Christmas tree.